Under certain growth conditions, some strains of Escherichia coli accumulate toxic levels of methylglyoxal. One such strain was isolated and shown to synthesize a mutant form of the cyclic AMP receptor protein and to have the gene for adenylate cyclase missing. During growth on glucose-6-phosphate, but not glucose there was premature growth arrest due to accumulation of methylglyoxal. The mechanism of this growth arrest is suggested to involve unbalanced metabolism through the glycolytic pathway with spillover of metabolites into a pathway leading to the formation of methylglyoxal.